1. Field of the Invention
The present invention relates to detection of the position of a magnetic pole in a rotor included in a synchronous motor, or the like, employing a permanent magnet.
2. Description of the Related Art
In a synchronous motor or a reluctance motor, a desired torque must be generated by causing a current to flow through phase windings according to the position of a magnetic pole in a rotor. The position of a magnetic pole in the rotor is therefore detected using an encoder or another sensor, so that the current will flow in the direction of an appropriate excitation phase. For this purpose, the position of a magnetic pole is detected at the time of starting up a motor in order to obviate the employment of a sensor having absolute values set therein for detection of the position of a magnetic pole or the work of aligning a sensor with a magnetic pole.
Moreover, as methods for detecting the position of a magnetic pole without employment of a sensor, there are methods including the one disclosed in, for example, Japanese Patent No. 3312520. According to the methods, a change in a ripple of an output of an inverter provided during each switching time under the control of a pulse-width modulator (PWM), or a change in an integrated value calculated by integrating values of an output voltage during the time is detected synchronously with the switching of the states of the inverter. The coordinates representing the position of a rotor are used to calculate the position of a magnetic pole according to a current-voltage equation relevant to a motor.
Moreover, according to methods including the one disclosed in Japanese Patent No. 3312472, an alternating voltage is applied to a motor, a detected current flowing through the motor is decomposed into a component parallel to the applied alternating voltage and a component orthogonal thereto, and the position of a magnetic pole is detected based on either of the parallel and orthogonal components.
Moreover, according to known methods including the one disclosed in Japanese Unexamined Patent Application Publication No. 2002-171798, a high-frequency voltage is superimposed on a direct-axis (d-axis) reference-wave voltage command, and the resultant direct-axis (d-axis) voltage command is transferred in order to decompose a current, which flows through an armature, into a direct-axis (d-axis) current and a quadrature-axis (q-axis) current. A high-frequency current whose frequency is identical to the frequency of the high-frequency voltage is sampled from the quadrature-axis (q-axis) current. The high-frequency current is used to calculate an estimated speed of a rotor and a first estimated position thereof. While the estimated speed and first estimated position are being calculated, pulsating voltages of positive and negative polarities are superimposed on the high-frequency voltage. Change rates, at which the direct-axis (d-axis) current changes along with superimpositions of positive and negative pulses, are used to calculate a second estimated position, that is, the position of a magnetic pole.
Furthermore, according to known methods including the one disclosed in Japanese Unexamined Patent Application Publication No. 2000-312493, the fact that an inductance decreases due to magnetic saturation is utilized in order to detect a d-axis current by applying positive and negative voltages of a certain level, which are regarded as vectors, to a motor by changing an angle at regular intervals. The initial point, of the vectors of the positive and negative voltages producing currents whose difference is the largest, is estimated as the position of a magnetic pole.
The foregoing methods of the related arts have a drawback that the position of a magnetic pole cannot be defected readily and accurately.